Field of Invention
The present invention relates to a transmission system and, in particular, to a transmission system having a subscriber line, such as an ISDN subscriber line.
FIG. 1 shows an ordinary transmission system or a circuit arrangement in which a signal transmission line 102, such as a two-wire ISDN subscriber line, is respectively isolated from a transmitter and from a receiver by means of a transformer 104, 106. The transformers 104 and 106 and the signal transmission line 102 form a four-terminal transmission network, which will be referred to several times below. The transmitter has an ideal voltage source 108 and an internal resistance (R0) 110. The ideal voltage source 108 with the source voltage U0 is connected to the transformer 104 via the internal resistance 110. At the reception end, the voltage UE is tapped off on a terminating resistance (R0) 112 and is processed further in the receiver. The internal resistance 110 of the ideal voltage source 108 is normally equal to the terminating resistance 112.
The insertion loss of the four-terminal transmission network is defined as the ratio of the voltage UE across the terminating resistance 112 to the voltage across the terminating resistance 112 in the absence of a four-terminal transmission network. The voltage across the terminating resistance 112 without a four-terminal transmission network is equal to half the source voltage U0. The following equation is thus obtained for the insertion loss:                               H          ⁡                      (            f            )                          =                                            U              E                        ⁡                          (              f              )                                                                          U                0                            ⁡                              (                f                )                                      /            2                                                           
FIG. 2 shows an example of the insertion loss of a four-terminal transmission network as shown in FIG. 1 when an ISDN access method is used. In this case, the ISDN signal transmission line has a length of 6.5 km and a diameter of 0.4 mm. The primary inductance of the transformers 104 and 106 has a value of 14 mH, and the terminating resistance 110 or the internal resistance 108 of the transmitter has a value of 135 Ω (ohms). By way of comparison, FIG. 6 also shows the pure line attenuation of the signal transmission line 102. It can be seen that the curve for the insertion loss differs significantly from the curve for the line attenuation only at low frequencies. The reason for this is that the characteristic impedance of the signal transmission line 102 differs relatively greatly from the terminating resistance 112 in this frequency range.
Subscriber access methods involve the use of a duplex-channel method with echo cancellation and pulse amplitude modulation (PAM) for speeds of up to 2.5 Mbit/s. The duplex-channel method is an electrical duplex transmission method for two-wire lines, in which the information in both directions of transmission is transmitted at the same frequency and in the same time slot. The purpose of echo cancellation is to render that component of the information from the transmitter which reaches the receiver inactive. The methods are described in more detail and specified, by way of example, in ANSI T1.601-1998 Integration Services Digital Network (ISDN)—Basic Access Interface for Use on Metallic Loops for Application on the Network Side of the NT (Layer 1 Specification), for the ISDN access method, and in ITU-T Q.4/15, “G.shdsl.bis: Draft Recommendation Text”, CS-R15, March 2002 for the SDSL method. On the basis of the pulse amplitude modulation, a transmission system involves almost complete use of the lower frequency range shown in FIG. 2, with limiting being provided merely by the transformers 104 and 106 (FIG. 1) used in the transmission system, the cutoff frequency of said transformers being dependent on the primary inductance and on the cutoff frequency of the high-pass filter used in the receiver. Since the line attenuation (shown in FIG. 2) of the signal transmission line 102 rises continuously with frequency, pulse amplitude modulation allows particularly good use to be made of the available bandwidth. With a given interference environment and a particular bit rate, however, the range which can be achieved for signal transmission is limited by the line attenuation. The range of signal transmission can then be increased only by reducing the bit rate or by using a repeater.
In the case of the ISDN access method, the range of signal transmission is normally increased, by way of example, by using a more efficient transmission method, e.g. SDSL, with corresponding coding, such as trellis coding. With an SDSL method, upon reaching the maximum range, a further increase is no longer possible, however, since more efficient transmission methods are currently still not available.